Studies have shown that individual rats of the same strain and sex vary in their response to standard doses of porphyrogenic chemicals. This variation is shown in the time it takes to respond by porphyrinuria, the time for porphyrin excretion to return to normal levels, and the intensity of the response. Some animals were found not to respond at all even after repeated doses of different porphyrogenic chemicals. Hexachlorobenzene (HCB) was the primary chemical on study because of its importance as a fungicide. Allyl isopropylacetamide (AIA) was used to study acute responses to rapid-acting porphyrogens. The response to AIA also varied among individual animals.